It is known to measure gas concentration by sensing the change in conductivity of an electrolyte when the gas dissolves in it. A conductimetric sensor is described by Himpler et al in Analytical Chemistry 50, No. 12, 1978, pp. 1623 to 1627. Gases which can be sensed in this way include carbon dioxide but a disadvantage of such a sensor is that the output has poor stability which results in base-line drift.
Another device which can be used to sense carbon dioxide concentration is the Stow-Severinghaus sensor in which the change in pH of an electrolyte due to dissolved CO.sub.2 is sensed by a glass electrode. Such a sensor, in a form suitable for transcutaneous measurement of CO.sub.2, is described in "Birth Defects": Original Article Series, Volume XV No. 4 pages 109 to 116, 1979. The National Foundation, in an article by D. Parker et al. A disadvantage of such a sensor is that miniature glass electrodes must be made individually by hand, and have a high source impedance which renders them susceptible to electrical noise and interference.